This invention relates to an A/D conversion circuit which converts analog values into digital values, and a solid-state image pickup device comprising an A/D conversion circuit.
A solid-state image pickup device comprises a plurality of photodetector elements, arranged in a one-dimensional or two-dimensional array, and a plurality of integrating circuits which integrate the signal currents output from each photodetector element to convert the signal currents into voltages. In this solid-state image pickup device, signal currents with values corresponding to the incident light intensity are output from the plurality of photodetector elements, voltages corresponding to the integrated signal currents are output from the integrating circuits, and based on the voltages, the distribution of the incident light intensity is obtained, to capture an image. A solid-state image pickup device may further comprise an A/D conversion circuit to convert voltages (analog values) output from the integrating circuits into digital values. In this case, an incident light intensity is obtained as a digital value, and image processing by a computer or similar becomes possible.
One well-known configuration of an A/D conversion circuit comprises N combinations of capacitors and switches; of the N capacitors, the nth capacitor has a capacitance value of 2nxe2x88x921 (Nxe2x89xa72, 1xe2x89xa6nxe2x89xa6N) . In this A/D conversion circuit, all capacitance values are set appropriately by appropriately setting the open/close states of each of the N switches according to the input analog value, so that an N-bit digital value corresponding to the open/close states of the N switches is output.
In such a solid-state image pickup device, faster operation and higher precision of the A/D conversion circuit are sought. In order to achieve faster operation, A/D conversion circuits are provided for each integrating circuit, to perform parallel processing. In order to increase precision, the number of bits of the digital value output from A/D conversion circuits is increased. Hence in order to improve both the processing speed and the precision of A/D conversion processing, an A/D conversion circuit may be provided for each integrating circuit to perform parallel processing, and the number of capacitors N in each A/D conversion circuit may be increased.
However, when attempting to fabricate a solid-state image pickup device comprising an A/D conversion circuit as described above on a single semiconductor chip, the following problems are encountered. The area occupied by capacitors on the chip is substantially proportional to the capacitance value. Hence if the number of bits is N, then the area occupied by N capacitors of an A/D conversion circuit is equivalent to the area occupied by one capacitor with capacitance value 2NC (≈C+2C+22C+. . . +2Nxe2x88x921C) . That is, if the number of bits is increased by one, the area occupied by all the capacitors of the A/D conversion circuit increases by a factor of 2. Also, if the capacitance value of a capacitor is large, the parasitic capacitance also increases, and fast A/D conversion processing becomes impossible. Hence conventional solid-state image pickup devices comprising A/D conversion circuits encounter limits in attempting to improve both speed, by providing an integrating circuit for each A/D conversion circuit, and precision, by increasing the number of bits of the digital values output from A/D conversion circuits.
The present invention was devised in order to resolve the above problems, and it is an object of the invention to provide an A/D conversion circuit which, while occupying a small area, easily achieves both fast operation and high precision, as well as a solid-state image pickup device comprising this A/D conversion circuit.
An A/D conversion circuit of this invention converts an analog value input to an input end into a digital value and outputs this digital value from an output end, and is characterized in comprising: (1) an amplifier, having a first input terminal, a second input terminal and an output terminal, in which the first input terminal is connected to the input end via a coupling capacitor, and a common voltage Vcom is input to the second input terminal; (2) a feedback capacitor, provided between the first input terminal and the output terminal of the amplifier; (3) a switch, provided between the first input terminal and the output terminal of the amplifier; (4) a number M of variable-capacitance portions, each having a number Nm of capacitors with different capacitance values, one end of each of which is connected to the first input terminal of the amplifier, and voltage switching means to switch the voltage input to each of the other ends of the Nm capacitors to a common voltage Vcom and to Pm reference voltages Vref,m,1 to Vref,m,Pm (Mxe2x89xa71, Nmxe2x89xa71, Pmxe2x89xa71, 1xe2x89xa6mxe2x89xa6M, but excluding the case M=P1=1); (5) a comparison portion, which compares the magnitudes of the voltage output from the amplifier output terminal and the common voltage Vcom, and outputs a signal indicating-the comparison result; and, (6) a capacitance control portion, which controls the switching operation in each of the voltage switching means of the M variable-capacitance portions, and which outputs a digital value to the output terminal based on the switched state in the respective voltage switching means of the M variable-capacitance portions as well as the signal output from the comparison portion.
By means of this A/D conversion circuit, when the switch between the first input terminal and the output terminal of the amplifier is closed, the feedback capacitor between the first input terminal and the output terminal of the amplifier is discharged. When the switch is then opened and a voltage (analog value) for A/D conversion is input from the input end, electric charge corresponding to the input voltage integrates across the feedback capacitor. Then, either the common voltage Vcom or one of the Pm reference voltages Vref,m,1 to Vref,m,Pm is switched by the voltage switching means to each of the other ends of the Nm capacitors comprised by each of the M variable capacitance portions controlled by the capacitance control portion. One end of the Nm capacitors comprised by each of the M variable capacitance portions is connected to the first input terminal of the amplifier, either directly or via a switch, so that upon switching, charge moves from the feedback capacitor to the M variable capacitance portions. Then, a voltage corresponding to the amount of charge remaining in the feedback capacitor is output from the output terminal of the amplifier. The magnitude of the voltage output from the amplifier is compared with the common voltage Vcom by the comparison portion, and a signal indicating the comparison result is output from the comparison portion to the capacitance control portion. Based on the switching states of the voltage switching means in each of the M variable capacitance portions and the signal output from the comparison portion, a digital value is output from the capacitance control portion to the output end.
In an A/D conversion circuit of this invention, it is preferable that M be 1, and that N1 and P1 be pluralities. In this case, the area occupied on the chip by the Nm capacitors comprised by each of the M variable capacitance portions is further reduced. In an A/D conversion circuit of this invention, it is preferable that M and N1 each be 1, and that P1 be a plurality; in this case, even compared with the above-described case, the area occupied on the chip by the Nm capacitors comprised by each of the M variable capacitance portions is further reduced.
A solid-state image pickup device of this invention is characterized in comprising (1) a photodetector element which outputs a signal current according to the incident light intensity; (2) an integrating circuit which inputs and integrates the signal current output from the photodetector element, and outputs a voltage corresponding to the integrated value of the signal current; and, (3) an A/D conversion circuit of this invention as described above, which inputs the voltage output from the integrating circuit, and converts this voltage into a digital value.
By means of this solid-state image pickup device, a signal current corresponding to the incident light intensity is output from the photodetector element, this signal current is input to and integrated by the integrating circuit, and a voltage corresponding to the integrated value is output from the integrating circuit. The voltage output from the integrating circuit is converted into a digital value by an A/D conversion circuit of this invention, described above.